It is known that hard vacuum pumps which operate on a molecular or a turbomolecular basis have rotors which rotate at high speed in stationary stators.
The molecule transport effect is set up at the periphery of the rotor as it rotates at high speed by reflecting a large proportion of these molecules in the zone of the stator which is adjacent to the rotor; the stator then returns a portion of the molecules to the part of the rotor located below and so on, such that the rotor and stator assembly has a high pressure ratio between the partial gas pressure at the discharge end and the pressure of the same gas at the suction end.
The need for a high rotor rotation speed gives rise to a serious problem in the case of bearings which must be installed in a chamber in which a vacuum is set up.
Known bearings lubricated with oil or grease operate in a primary vacuum and a known motor is used which also operates in a vacuum. However, pumps thus equipped have a number of disadvantages:
In particular, it is difficult to lubricate the ball bearings and other related components properly and to cool them sufficiently.
Further, it is practically impossible to prevent oil and grease from rising up to the pump suction zone level.
Published French patent application No. 2 446 934 describes a hard vacuum pump which makes it possible to remedy these difficulties.
It includes a rotor driven by a motor located outside the pumping chamber and fixed thereto by a drive shaft sealed by a grooved dynamic molecular seal. In said pump, the shaft is supported firstly by an outer bearing located outside the pumping chamber and secondly by an inner bearing located inside the pumping chamber and lubricated with grease.
The inner bearing is located between the grooved dynamic molecular seal and a viscous dynamic molecular seal interposed between the inner bearing and the outer bearing. A longitudinal groove makes the two surfaces of the inner bearing communicate with each other.
This solves the problem of cooling the bearings and the drive motor.
Indeed, the ball bearings located in the empty zone are easily cooled by conduction along the shaft, one of whose ends is easily cooled since it is in contact with the outside atmosphere. Likewise, the rotor and the stator of the motor are cooled by conventional means since these two components are in contact with the outside atmosphere, as is the outer bearing.
However, when the pumped gases are particularly corrosive, the position of the ball bearings relative to the flow of pumped gas is not very good despite the location of the longitudinal groove which prevents gas from flowing directly through the bearing and further, since one end of the movable assembly is located in a very low pressure chamber and the other end thereof is subjected to atmospheric pressure, a high axial load results therefrom and is borne by one of the two bearings. This limits the service life of the pump.
Preferred embodiments of the present invention mitigate these disadvantages while also providing proper cooling of the bearings and of the motor.